Update pkgdown mode and vignettes output

[joshwlambert]

This PR updates the vignettes to use rmarkdown::html_document() rather than bookdown::html_document2() due to issues with compatibility after the release of {pkgdown} v2.1.0.

As a result of the new vignette output the as_is: true has been removed, as has code folding which is not an option with rmarkdown::html_document().

The development: mode has been changed in _pkgdown.yml to unreleased from auto until the package is on CRAN (epiverse-trace/blueprints#100).

As {bookdown} is no longer used by the package it is removed from Suggests in the DESCRIPTION.

[github-actions]

This pull request:

• Adds 0 new dependencies (direct and indirect)
• Adds 0 new system dependencies
• Removes 1 existing dependencies (direct and indirect)
• Removes 1 existing system dependencies

(Note that results may be inacurrate if you branched from an outdated version of the target branch.)

[github-actions]

This pull request:

• Adds 0 new dependencies (direct and indirect)
• Adds 0 new system dependencies
• Removes 1 existing dependencies (direct and indirect)
• Removes 1 existing system dependencies

(Note that results may be inacurrate if you branched from an outdated version of the target branch.)